This invention relates to a semiconductor device comprising a thyristor and especially, but not exclusively, to a semiconductor device comprising a so-called MOS-gated thyristor.
Semiconductor devices are known which comprise a semiconductor body having formed therein a thyristor having a first region of one conductivity type provided with a first main electrode, a second region of the opposite conductivity type forming a first pn junction with the first region, a third region of the one conductivity type forming a second pn junction with the second region and provided with a gate electrode, and a fourth region of the opposite conductivity type forming a third pn junction with the third region and having an electrical connection to a second main electrode.
In particular, semiconductor devices are known in which the gate electrode is in the form of an insulated gate overlying a conduction channel area of the third region for controlling a conductive path between the second region and the fourth region to enable the flow of charge carriers of the opposite conductivity type from the fourth region to the second region to trigger latching and so initiate thyristor action within the device. Such a MOS-gated thyristor is described at page 411 of a review entitled `Evolution of MOS-bipolar Power Semiconductor Technology` by B. Jayant Baliga published in Proceedings of the IEEE Vol. 76, No. 4, Apr., 1988.
The aforementioned paper also describes at pages 415 to 416 a so-called MOS turn-off thyristor which differs from the simple MOS-gated thyristor in that it is capable of being both turned-on and turned-off by a signal applied to a MOS gate. The MOS turn-off thyristor consists of an insulated gate field effect transistor (MOST) integrated into the thyristor structure in such a manner that the emitter-base junction of the upper transistor can be short-circuited by the application of a gate voltage to the MOST. In the absence of the gate voltage, the device can be switched on either in the same manner as a conventional thyristor or by using a MOS gate in the same manner as for a MOS-gated thyristor. Thus, as shown in FIG. 15 of the aforementioned paper, the MOS turn-off thyristor has a fifth region of the one conductivity type formed within and shorted to the fourth region and the contiguous insulated gate overlies channel areas of the fourth and third regions. In order to achieve turn-off of the thyristor, the MOST defined between the fifth and third regions is turned-on by an appropriate voltage applied to the insulated gate so that charge carriers of the one conductivity type entering the third region have an alternate path to the second main electrode by-passing the pn junction between the fourth and third regions. However to achieve such forced turn-off, the resistance of the current path for the charge carriers of the one conductivity type must be so low that, when all such current is diverted via the MOST between the fifth and third regions, the forward biassing of the third pn junction is below 0.7 volts and so insufficient to maintain electron injection and transistor action. Although this condition may be quite readily achieved for relatively high current densities where the charge carriers of the one conductivity type are electrons, it is more of a problem where the charge carriers are less mobile holes.